In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy, for example, to heat. The absorption of light during wave propagation is often called attenuation. Usually, the absorption of waves does not depend on their intensity (linear absorption), although in certain conditions (usually, in optics), the medium changes its transparency dependently on the intensity of waves going through, and the Saturable absorption (or nonlinear absorption) occurs.

## Quantifying absorption

There are a number of ways to quantify how quickly and effectively radiation is absorbed in a certain medium, for example:

• Absorption coefficient, and some closely-related derived quantities:
• Attenuation coefficient, which is sometimes but not always synonymous with absorption coefficient
• Molar absorptivity, also called "molar extinction coefficient", which is the absorption coefficient divided by molarity (see also Beer-Lambert law).
• Mass attenuation coefficient, also called "mass extinction coefficient", which is the absorption coefficient divided by density; see mass attenuation coefficient for details.
• Absorption cross section and scattering cross section are closely related to absorption coefficient and attenuation coefficient, respectively.
• "Extinction" in astronomy is equivalent to the attenuation coefficient.
• Penetration depth and Skin depth,
• Propagation constant, attenuation constant, phase constant, and complex wavenumber,
• Complex refractive index and extinction coefficient,
• Complex dielectric constant,
• AC conductivity.
• Absorbance (also called "optical density") and optical thickness (also called "optical depth") are two related measures of the total light-blocking power of a certain medium with a certain thickness.
• Percentage of the incoming light which gets absorbed

All these quantities measure, at least to some extent, the same thing: How well a medium absorbs radiation. However, practitioners of different fields and techniques tend to conventionally use different quantities drawn from the list above. Fortunately, it is easy to convert from one measure to another, see Mathematical descriptions of opacity.

## Measuring absorption

The absorbance of an object quantifies how much of the incident light is absorbed by it (not all photons get absorbed, some are reflected or refracted instead). This may be related to other properties of the object through the Beer-Lambert law.

Precise measurements of the absorbance at many wavelengths allow the identification of a substance via absorption spectroscopy, where a sample is illuminated from one side, and the intensity of the light that exits from the sample in every direction is measured. A few examples of absorption spectroscopy, in different parts of the spectrum, are ultraviolet-visible spectroscopy, infrared spectroscopy, and X-ray absorption spectroscopy.

## Applications

Understanding and measuring the absorption of electromagnetic radiation has an enormous variety of applications. Here are just a few examples:

• In meteorology and climatology, global and local temperatures depend in large part on the absorption of radiation by atmospheric gases (such as in the greenhouse effect) and the ground (see albedo).
• In medicine, X-rays are absorbed to different extents by different tissues (bone in particular), which is the basis for X-ray imaging.
• In chemistry and materials science, different materials and molecules will absorb radiation to different extents at different frequencies, which allows for material identification.
• In optics, sunglasses, colored filters, dyes, and other such materials are designed specifically with respect to which visible wavelengths they absorb and how much.
• In biology, photosynthetic organisms require that light of the appropriate wavelengths be absorbed within the active area of chloroplasts, so that the light energy can be converted into chemical energy within sugars and other molecules.